1. Field of the invention
The present invention relates, in general, to a process for preparing metal oxide slurries for use in chemical mechanical polishing (CMP) of semiconductor devices and, more particularly, to the use of two complementary intensifier pumps in the dispersion of metal oxide slurries to maintain the pressure applied to the dispersion chamber constantly, thereby making the metal oxide slurries nearly free of macro particles.
2. Description of the Prior Art
To better achieve a lithography in the fabrication of semiconductor devices which have been gradually miniaturized and being of high integration and multi-layer structures, the CMP process is now indispensably used to conduct planarization over a wide area of which conventional spin-on-glass (SOG) or etchback techniques cannot take care. In the CMP process, a substrate is usually immersed in metal oxide slurry in conjunction with an elastomeric pad which is pressed against the substrate and rotated such that the slurry particles move across the substrate surface to accomplish wear and volumetric removal of the substrate surface.
It is naturally required for the metal oxide slurry used in the CMP process to be superior in dispersion stability and to be of high polishing rate and high purity as well as not to leave defects, such as microscratches, on the surface of the wafer after polishing. If occurring during the CMP process, the defects, such as microscratches, bring about a fatal consequence in the function and production yield of semiconductor devices.
The above requirements, except for the requirement of being of high purity, are in close connection to the particle size and size distribution of metal oxides, which are main components of the polishing slurries. For example, in regard to the particle size, larger particle sizes of metal oxides give a greater contribution to polishing rate, but result in poorer dispersion stability of slurries, causing the sedimentation of the particles and thus, requiring a stirring process prior to use. On the other hand, as the metal oxides are of smaller particle sizes, the dispersion stability becomes better with less occurrence in the frequency of microscratches. However, the rate of surface removal is reduced. As for the size distribution, the metal oxides are preferably uniform in particle size. For example, after being treated with a slurry which is in a broad range of particle size distributions, the substrate surface shows indigent flatness and is let to undergo the occurrence of a significant quantity of microscratches. Therefore, when taking account of polishing rate, dispersion stability, and microscratch occurrence, polishing slurries for the CMP of semiconductor devices preferably contain metal oxides with an appropriate range of particle sizes and size distributions.
Also important is the purity requirement for the slurries. Where the metal oxide slurries contain metal ions, such as sodium, they diffuse in the wafer, deteriorating the production yield of the semiconductors.
A conventional process for preparing CMP metal oxide slurries can be referred to U.S. Pat. No. 5,383,372 in which a metal oxide is agitated in a mixture with a chemical additive such as an alkaline material or an oxidant, in the presence of beads by use of a dynomill or ballmill. This process, however, suffers from significant problems in that the dispersion mechanism utilizing the impact of the beads on the mixture cannot avoid the pollution of the bead component as well as causes a tailing phenomenon in particle size distribution, making it difficult for metal oxide slurries to have uniform particle sizes. In addition, the bead friction lowers the dispersibility of the mixture with the lapse of time, aggravating the deviation of particle size and distribution by lots. Thus, it is difficult to reproduce identical polishing ability.
Another process for preparing CMP metal oxide slurries is disclosed by IKA, Germany, in which a rotor is rotated at a high speed to cause collision and friction against a stator. Although advantageous over the above-referred patent, this process still shows the same problems: the etching due to the collision against the stator and the dispersibility attenuation with the lapse of time.
In addition, the above-referenced processes both can reduce abrasive particles only to the size of about 1 xcexcm, which is too large to be used suitably for CMP. Particularly, with feasibility to cause microscratches, such large abrasive particles are stoutly excluded from CMP slurries for use in shallow trench isolation (STI) as this process is fatally affected, in conjunction with the function and yield of semiconductor devices, by scratches.
Korean Patent Application No. 98-39212, yielded to the present inventors, discloses a novel process in which a fluid is accelerated at a high pressure to cause a cooperation of shearing force, impact and cavitation at an orifice of a dispersion chamber, through which metal oxide slurries are dispersed when passing. Simpler in procedure than and far superior in particle size distribution and polishing rate to conventional processes as it is, the previous process of the present inventors still needs to be improved in regard to microscratch occurrence.
Leading to the present invention, the intensive and thorough research, repeated by the present inventors, on the relation of main parameters to the generation of macro-particles (as large as or larger than 1 xcexcm in size) when metal oxide slurries are prepared by dispersion through an orifice at which a fluid is accelerated under a high pressure, resulted in the finding that the macro-particles can be prevented from being generated by controlling the pressure profile of the high pressure pump.
It is an object of the present invention to overcome the above problems encountered in prior arts and to provide a process for preparing metal oxide slurries, which are almost so exclusive of macro-particles as large as 1 xcexcm in size as to be suitable for the CMP of semiconductor devices.
In accordance with the present invention, there is provided a process for preparing metal oxide slurries suitable for the chemical mechanical polishing of semiconductor devices, in which a suspension of metal oxide in water is dispersed at a predetermined pressure through an orifice of a dispersion chamber while two intensifier pumps are used to maintain the pressure applied to the dispersion chamber constantly.